1. Field of the Invention
The present invention relates to a pulsed arc welding method using, as a shield gas, carbon dioxide gas alone or a mixed gas containing a carbon dioxide gas as a main component, more particularly, to a pulsed arc welding method in which droplet transfer is realized in synchronism with a group of pulses to thereby stabilize a welding arc and, at the same time, to significantly reduce spatter and fume generation rates.
2. Description of the Related Art
The MAG welding method using, as a shield gas, a mixed gas of Ar and 5 to 30% of CO2 is able to reduce spatter and fume generation rates through the fine droplets, for which the method has been applied to a wide variety of fields in the past. Especially, in the field where high quality welding is required, the pulse MAG welding method wherein one pulse-one droplet transfer is performed by outputting a welding current of about 100 to 350 Hz as a pulse current has now been in wide applications.
However, since Ar gas is costly, compared with carbon dioxide gas, carbon dioxide gas alone or a mixed gas made mainly of carbon dioxide gas has been predominantly used as a shield gas for carrying out welding operations in general.
On the other hand, in the case that carbon dioxide gas alone or a mixed gas made mainly of carbon dioxide gas is used as a shield gas, the resulting droplet is rendered coarse in size to an extent of about 10 times larger over the case of the MAG welding method and is irregularly vibrated and deformed by the action of the arc force. This undesirably leads to the problems in that short-circuiting with a base metal and arc break are liable to occur, droplet transfer becomes irregular, and spatters and fumes are excessively generated.
To cope with such problems, Japanese Patent Laid-Open Nos. Hei 7-290241 and Hei 7-47473 propose a method for realizing a one pulse-one droplet transfer even in the carbon dioxide gas arc welding by applying pulse welding to carbon dioxide gas shield arc welding and by defining pulse parameters and welding wire components. According to this technique in the art, a droplet of a satisfactory size is formed at a wire tip prior to application of a peak current so that an electromagnetic pinch force of the peak current may cause the droplet to be constricted at an early stage, thereby permitting the droplet to be released from the wire before the droplet is forced back toward the wire direction by the arc force.
In regard to the above welding method, Japanese Patent Laid-Open No. Hei 8-267238 has proposed a welding method wherein external output-characteristics change control is performed for controlling output of power source for welding, thereby achieving a further reduction of spatter.
Moreover, Japanese Patent Laid-Open Nos. 2003-236668 and 2001-129668 relate to an arc welding method using a shield gas made mainly of carbon dioxide gas, wherein it is stated that generation of seven or more pulses within one droplet transfer time contributes to reducing spatters and weld fumes.
Further, Japanese Laid-Open Patent Application No. Hei 8-229680 relates to an output control device of a pulse arc welding machine using a shield gas made mainly of carbon dioxide gas, wherein it is stated that the release of a droplet is detected by an increase of voltage or resistance and the current of a lower level is outputted for a predetermined period of time from the detection time, to thereby suppress the generation of spatters.
In addition, Japanese Laid-Open Patent Application No. Hei 10-263815 states that the generation of spatters can be suppressed by utilizing a pulse arc welding machine which outputs two different pulse waveforms using a shield gas made mainly of carbon dioxide gas, wherein the pulse waveforms comprise a first pulse setting a pulse period and a base period short according to an increase in the wire feed quantity and a second pulse with a pulse period shorter than the first pulse.
The conventional welding methods described in the above Japanese Patent Laid-Open Nos. Hei 7-290241, Hei 7-47473, and Hei 8-267238 make use of inexpensive carbon dioxide gas as a shield gas, enable one pulse-one droplet transfer and improve regularity of the droplet transfer. At the same time, a large-particle spatter generation rate can be reduced over pulse-free welding. In these conventional methods, however, there is a problem that the droplet is released during the pulse peak period, causing particulate spatters to be generated in large amounts due to scattering in the constricted portion of the wire tip when the droplet is released and due to scattering of a molten pool remaining on the wire after the droplet is released therefrom.
In the methods disclosed in Japanese Patent Laid-Open Nos. 2003-236668 and 2001-129668, it is stated that when seven or more pulses are oscillated within one droplet transfer time, droplets can be made small in size. Nevertheless, as long as a gas made mainly of carbon dioxide gas is used as a shield gas, the size of a droplet is at least 10 times the size of a droplet in the MAG pulse welding, with the particulation effect being not so significant. The droplet transfer is complicatedly interrelated with the size of a droplet, the electromagnetic pinch force in a peak period, an upward force resulting from an arc force, convection and vibrations inside the droplet ascribed to these factors, and the like. The release timing is determined through the balance of a force acting along a release direction of the droplet. Therefore, a significant reduction of spatters cannot be attained by simply applying high frequency pulses continuously as in these conventional methods because the release time differs in every release timing, and the intervals of the droplet transfer vary within a range of about 15 to 25 milliseconds.
Moreover, in the above methods, since a high frequency pulse is applied simply to ensure smooth droplet transfer and thus, a peak current, base current and pulse width are, respectively, fixed, a frequency has to be modulated for the purpose of controlling an arc length at a given level in the case where the distance between a chip and a base metal is varied. That is, in order to control a wire melting rate, a pulse frequency has to be greatly changed, but this only disturbs regularity of droplet transfer. Accordingly, in the case of weaving a groove where the distance between a chip and a base metal varies within about ±5 mm from a standard condition, a difficulty is involved in keeping a stable arc.
Although Japanese Laid-Open Patent Application No. Hei 8-229680 teaches that the output control device is capable of suppressing the generation of spatters by detecting the release of a droplet and lowering the amount of current for a given period, it is based on an assumption that a pulse peak current is constant for every pulse regardless of whether or not a droplet has been released. If a pulse peak current permitting the release of a droplet is set, a molten metal remaining on a wire after the release of a droplet scatters by a great arc force when a subsequent pulse peak after the release is applied and generates a lot of large-particle spatters. Lowering the pulse peak current to suppress this only causes the droplet not to be released during the pulse peak period.
Further, as described above, according to Japanese Laid-Open Patent Application No. Hei 10-263815, the generation of spatters can be suppressed by a pulse arc welding method outputting two different pulse waveforms, wherein the pulse waveforms comprise a first pulse setting a pulse period and a base period short according to an increase in the wire feed quantity and a second pulse with a pulse period shorter than the first pulse. However, when a first pulse period and a first base period are set short according to an increase in the wire feed quantity, in a cutoff layer under an electromagnetic pinch force by the second pulse, a droplet at a wire tip is not shaped in a satisfactory manner and the electromagnetic pinch force is not effectively applied. In consequence, regularity of one pulse group-one droplet transfer cannot be kept, and large-particle spatters are generated.